Lightguide fibers, or optical fibers, respond to certain temperature or pressure variations by producing small corresponding changes in the phase of light signals being transmitted therethrough. Those phase changes are effective changes in the signal path length due to the index of refraction change in the lightguide fiber. It is useful to induce effective path length changes in order to detect pressure changes representing effects that may be of interest; but signals which induce those changes are often so small that they may be at least partly masked by the sensitivity fluctuation of the interferometer due to the ambient, or environmental, temperature or pressure effects varying at a relatively low frequency compared to the frequency of signals which are to be detected.
P. Shajenko et al. propose in "Signal Stabilization of Optical Interferometric Hydrophones by Tuning the Light Source," Applied Optics, Vol. 19, No. 12, June 15, 1980, pages 1895-1897, a laser-driven optical interferometer. That system employs a stabilization feedback loop for adaptively tuning the laser to compensate for instability tendencies caused by low frequency, thermal and static, pressure fluctuations. Moving parts are required for changing the cavity size of the helium-neon laser that is used in the proposal, and the entire apparatus is relatively large and cumbersome.
An implementation of the Shajenko et al. proposal using an argon-pumped rhodamine 6G dye laser with an electrooptic tuner and a Michelson interferometer is disclosed in "Active Stabilization of a Michelson Interferometer by an Electrooptically Tuned Laser," by A. Olsson et al., Applied Optics, Vol. 19, No. 12, June 15, 1980, pages 1897-1899. A feedback signal from the interferometer, at the same frequency as a modulation signal applied to the laser, is used to tune the laser over a range much larger than that of the laser contemplated by Shajenko et al., but again the apparatus is large and cumbersome. Also, typical dye laser lifetime is less than a year.
Physical size is an important consideration in many measuring and sensing applications such as manufacturing operations, or operation of robotic equipment, or undersea geophysical prospecting, in all of which it is necessary to detect minute displacements in small, difficult to reach locations. In these kinds of applications it is often advantageous to have an entire sensing apparatus close to a sensing head so that a local microprocessor can perform necessary measured signal processing and transmit only final result signals back to a central control site. Apart from size, electric power requirements of measuring apparatus are also important considerations. Laser apparatus in each of the aforementioned systems is too large to be incorporated into a small, difficult to reach, measuring situation, and each consumes so much power that heat removal may be a problem in confined locations.
Semiconductor diode lasers of the single section type are much smaller, and consume less power, than the foregoing types of lasers and are conveniently tunable by modification of laser bias current. Examples of such tunable lasers are mentioned in "Direct Frequency Modulation in AlGaAs Semiconductor Laser," by S. Kobayashi et al., IEEE Journal of Quantum Electronics, Vol. QE-18, No. 4, 1982, pages 582-595. Although a semiconductor laser measuring system could be packaged in a much smaller size than the other laser systems previously mentioned (because the semiconductor laser itself is only the size of a pin head) and would consume only about 10.sup.-3 as much power, such lasers are known to be characterized by a short coherence length, e.g., one millimeter or less, which is too short to be used in a practical interferometric sensor.
In "High-Speed Direct Single-Frequency Modulation With Large Tuning Rate and Frequency Excursion in Cleaved-Coupled-Cavity Semiconductor Laser" by W. T. Tsang et al., Applied Physics Letters, Vol. 42, No. 8, Apr. 15, 1983, pages 650-652, the tunability of the so-called C.sup.3 laser is discussed. Changes of current in a modulation section of the laser, cause the frequency of the laser to change; and a corresponding change in wavelength occurs. A tuning range of about half of the spectral width of the gain profile of the laser results, i.e., about 4 kilogigahertz (kGHz) at 1.5 .mu.m wavelength. Frequency modulator applications were contemplated by the authors.
A C.sup.3 laser is stabilized against slow environmental fluctuations and changes due to normal aging of the laser diode as described by N. A. Olsson et al. in "Active Spectral Stabilization of Cleaved-Coupled-Cavity (C.sup.3) Lasers," Journal of Lightwave Technology, Vol. LT-2, No. 1, February 1984, pages 49-51. A stabilization feedback loop is applied to the modulator section of the laser to increase the ranges of temperature and ranges of laser section drive current for which the laser can be operated in a stable single mode, i.e., without mode hopping.